When a strong monochromatic wave is incident on a transparent medium, it causes waves at lower frequencies to experience exponential gain if their frequency offset corresponds to the frequency of some excitation in the medium. Stimulated scattering of optical beams is such an excitation, whether it be of Raman, Brillouin, Rayleigh, or other origin, and the dominant backscattering has a very large phase conjugate component. Thus, these effects have been used as conjugate wave generators or phase-conjugate mirrors.
In conjugate wave generation, a new wave can be created when an incident wave impinges on a transparent medium. The new wave travels in a direction opposite to and with the opposite sign of transverse phase to the incident waves phase. Therefore, a diverging wave that is incident on a conjugator will leave as a converging wave that retraces the same path as the incident wave. Thus, a phase conjugator can be considered as a unique mirror that combines reflection with phase reversal.
Initial reports of optical phase conjugation have been published by workers in the Soviet Union in 1971 and 1972 but little or no interest was created in the subject until more recent years when potential applications for phase conjugation began to appear in the open literature. Of these applications most activity has centered on applications to adaptive optics where the effects of atmospheric turbulence on high-energy laser radiation may be corrected and where optical train distortions and pointing errors may be corrected; and on applications to optical resonators where defraction limited output beams may be produced, even though the active medium may be causing distortions. Some of the results of this activity and an explanation of phase conjugate mirrors are given in a review paper entitled "Optical Beam Phase Conjugation by Stimulated Backscattering" by R. W. Hellwarth, Optical Engineering, Vol. 21, No. 2, March/April 1982, pages 257-262; and in a paper entitled "Can Phase Conjugate Resonators Enhance Laser Performance" by C. R. Giuliano, et al., Laser Focus, Vol. 2, February 1983, pages 55-64. Phase conjugate mirrors are now in existence for radiation from some lasers, but do not exist for radiation from the high power 10.6 .mu.m laser.